Transmission output speed retarders are non-wearing auxiliary braking devices used in conjunction with a rotatable vehicle transmission output member or driveshaft in order to safely augment the conventional friction-based braking system or service brakes used on certain large vehicles, such as diesel-powered medium and heavy trucks or busses. Such speed retarding devices or retarders are useful in helping to slow or stop a vehicle, particularly under continuous-braking or start-stop conditions and while the vehicle is descending a relatively long, steep slope, such as a descending stretch of mountain highway. Without the use of a transmission output speed retarder, a conventional vehicle braking system operating continuously under such aggressive slope or traffic conditions may tend to wear more rapidly, potentially reducing the working life of the service brake.
Because of their added safety and maintenance benefits, retarder devices of varying designs or styles are popular transmission add-ons or accessories. Hydrodynamic and electrodynamic retarders are two of the more commonly used retarder devices. Hydrodynamic/hydraulic retarders circulate pressurized fluid within a rotor that is enclosed within a separate, vaned stationary housing in order to induce a viscous drag by way of an opposing fluid coupling effect, thus slowing the rotating driveshaft in proportion to the fluid pressure and/or flow, i.e. the retarder request. Likewise, electrodynamic/electric retarders produce a magnetic field in an opposite rotational direction to that of the driveshaft, thus slowing the vehicle. Other retarder methods or devices may also be used to slow a vehicle, such as exhaust brakes, engine brakes, or Jake Brakes, which act to load a vehicle engine and thereby slow its rate of rotation.
Various operator-directed control systems exist for the purpose of controlling a fixed amount of retarder capacity to be applied to the vehicle transmission. For example, an operator-actuated lever, switch, and/or brake pedal may be used to command a predetermined amount of retarder request based on, for example, a predetermined percentage of available retarder capacity or retarder torque. However, such devices may be less than optimal, as they generally require an operator to actuate a retarder mechanism each time the operator wishes to engage the retarder. Likewise, such mechanisms may provide inadequate vehicle speed control, as a given vehicle's rate of deceleration will necessarily vary along with its gross weight and/or axle ratio as the vehicle travels over different terrain and through various traffic conditions, thus requiring frequent retarder adjustments in order to maintain even a generally constant speed.